Marine seismic surveys are usually conducted by towing an energy source and seismic detectors behind a vessel. The source imparts an acoustic wave to the water, creating a wavefield which travels coherently into the underlying earth. As the wavefield strikes interfaces between earth formations, or strata, it is reflected back through the earth and water to the detectors, where it is converted to electrical signals and recorded. Through analysis of these signals, it is possible to determine the shape, position, and lithology of the sub-bottom formations.
In other marine survey methods, the detectors and/or sources are placed at or close to the sea bottom, such as ocean bottom cables (OBCs) or in wells drilled into the sea bottom.
A seismic wave generated in (or reflected of) earth strata passes into the water in a generally upward direction. The wave that travels only once upwards, termed the “primary”, travels through the water and past the seismic detector which records its presence. The wavefield then continues to travel upward to the water's surface (which can be regarded as a free surface), where it is reflected back downwards. This reflected, or “ghost”, wavefield also travels through the water and past the detector(s), where it is again recorded. These down-going ghost reflections are an undesirable source of contamination of seismic data, since they obscure the interpretation of the desired up-going reflections from the earth's interior.
As described for example in the published international patent application WO-03/058281 ghost reflections or events can be regarded as being filtered or transformed versions of the direct signalD=G U  [1]where D is the down-going wavefield, U is the desired up-going wavefield and G is the ghost filter or ghost operator that emulates the effects of the reflecting surface such as additional traveltime (or depth of the receiver or source below the reflecting surface) and phase changes. Knowledge of G or an approximation thereof, allows an operator to remove ghost from the recorded data using subtraction and/or an inverse operation.
In addition to generating ghosts, the free surface also gives rise to a series of one or more subsequent reflections or multiples that are reflection of ghost events in the deeper layers of the earth. In contrast to ghosts these events are registered in the up-going wavefield and are, hence, not separable from the desired primary event through a separation of the recorded wavefield into up- and down-going.
Free-surface multiple reflections can be classified according to their order, which is equal to the number of reflections from the free surface, e.g. first (second) order free-surface multiple reflections undergo one (two) downward reflections from the sea-surface before being detected as up-going by the hydrophones, and so on. Of course, the multiples themselves generate ghost events in the recorded data.
The problems relating to the removal of ghosts (deghosting) and of multiples attracted a lot of efforts in the industry and generated a large body of literature, of which only a small amount can be reasonably cited. For deghosting methods, reference is made for example to the above WO-03/058281, further to WO-02/01254 and U.S. Pat. No. 6,529,445 and GB patent applications 2363459 and 2379741.
Multiple removal methods are described for example in U.S. Pat. Nos. 5,757,723, 5,587,965, 5,995,905 and 6,101,448.
A recent publication by Calvert and Will presented as paper A15 at the 65th EAGE Conference June 2003 proposes to use a traveltime filter to suppress first order multiples in a time lapse seismic survey using an ocean bottom cable with sparsely distributed receivers to monitor changes in a subsurface reservoir.
It is an object of the present invention to provide a method for removing multiples from recorded or acquired marine seismic data.